It was during Thrór's reign that Smaug descended on Lonely Mountain and drove the Dwarves into exile. Long after his kingdom's destruction, Thrór passed the ring to his son Thráin, who dwelt for many years as an exile from his ancient home.

At last, Thráin set out on an ill-fated quest to regain his kingdom, but he was captured by the spies of Sauron, and the Ring of Thrór was lost forever.

Many Dwarves however did not know the fate of the Ring, and thought that it was lost in Moria when Thrór was killed by Azog. It was thought that one of the reasons Balin wished to reclaim Moria was to find the Ring. In the Council of Elrond, Gandalf told Glóin that Balin would not find the Ring there, as it was given to Thráin and lost in Dol Guldur.[3]